<!DOCTYPE html>

<html lang="en">

<head>

  <meta charset="UTF-8">

  <title>Code</title>

  <style>

    body{

      margin: 0;

      overflow: hidden;

    }

  </style>

</head>

<body>

<canvas id="myCanvas"></canvas>

<script>

  const width = document.getElementById("myCanvas").width = screen.availWidth;

  const height = document.getElementById("myCanvas").height = screen.availHeight;

  const ctx = document.getElementById("myCanvas").getContext("2d");

  const arr = Array(Math.ceil(width / 10)).fill(0);

  const str = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".split("");

  function rain() {

    ctx.fillStyle = "rgba(0,0,0,0.05)";

    ctx.fillRect(0, 0, width, height);

    ctx.fillStyle = "#0f0";

    arr.forEach(function (value, index) {

      ctx.fillText(str[Math.floor(Math.random() * str.length)], index * 10, value + 10);

      arr[index] = value >= height || value > 8888 * Math.random() ? 0 : value + 10;

    });

  }

  setInterval(rain, 30);

</script>

</body>

</html>